Processing of semiconductor substrates or wafers presents unique problems not associated with processing of other workpieces. Typically, the semiconductor processing begins with a silicon wafer. The semiconductor processing starts with doping of the silicon wafer to produce transistors. Next, the semiconductor processing continues with deposition of metal and dielectric layers interspersed with etching of lines and vias to produce transistor contacts and interconnect structures. Ultimately in semiconductor processing, the transistors, the transistor contacts, and the interconnects form integrated circuits.
A critical processing requirement for the processing of the semiconductor wafer is cleanliness. Much of semiconductor processing takes place in vacuum, which is an inherently clean environment. Other semiconductor processing takes place in a wet process at atmospheric pressure, which because of a rinsing nature of the wet process is an inherently clean process. For example, removal of photoresist and photoresist residue subsequent to etching of the lines and the vias uses plasma ashing, a vacuum process, followed by stripping in a stripper bath, a wet process.
Other critical processing requirements for the processing of the semiconductor wafers include throughput and reliability. Production processing of the semiconductor wafers takes place in a semiconductor fabrication facility. The semiconductor fabrication facility requires a large capital outlay for processing equipment, for the facility itself, and for a staff to run it. In order to recoup these expenses and generate a sufficient income from the facility, the processing equipment requires a throughput of a sufficient number of the wafers in a period of time. The processing equipment must also promote a reliable process in order to ensure continued revenue from the facility.
Until recently, the plasma ashing and the stripper bath were found sufficient for the removal of the photoresist and the photoresist residue in the semiconductor processing. However, recent advancements for the integrated circuits have made the plasma ashing and the stripper bath untenable for highly advanced integrated circuits. These recent advancements include small critical dimensions for etch features and low dielectric constant materials for insulators. The small critical dimensions for the etch features result in insufficient structure for lines to withstand the stripper bath leading to a need for a replacement for the stripper bath. Many of the low dielectric constant materials cannot withstand an oxygen environment of the plasma ashing leading to a need for a replacement for the plasma ashing.
Recently, interest has developed in replacing the plasma ashing and the stripper bath for the removal of the photoresist and the photoresist residue with a supercritical process. The methods for holding semiconductor wafers in position during processing are well known in the art. The physical environment in which the wafer is subjected is the largest determining factor as to which method to use to restrain the wafer during processing. The use of high pressure and high temperature carbon dioxide is fairly new to the semiconductor processing arena. High pressure processing chambers of existing supercritical processing systems are not appropriate to meet the unique needs of the semiconductor processing. In particular, high pressure chambers of existing supercritical processing systems do not provide a device for securing the semiconductor wafer during the supercritical processing that does not break the semiconductor wafers or otherwise damage the semiconductor wafer.
Vacuum has been used in many different semiconductor equipment types to hold the wafer to a metal “chuck” for processing. In certain types of vacuum chucks, a vacuum groove is used to hold the semiconductor wafer to the semiconductor holding region. In these types of vacuum chucks, the underside of a semiconductor wafer has a sufficient roughness which allows leakage to occur between the underside of the wafer and the extremely smooth chuck surface. The leakage between the wafer and the chuck surface poses a problem, because the wafer is not sufficiently held on the chuck surface as the supercritical process begins.
In addition, it has been found that a vacuum groove upon which the semiconductor wafer spans creates excessively high stresses in the wafer when subjected to the high pressure environment. Based upon the strength of a wafer, either the width of the vacuum groove must be reduced to eliminate the possibility of damage to the wafer or the external force pushing the wafer into the vacuum groove (gas pressure) must be reduced. In order to perform the necessary processing on the wafer, the pressure cannot be reduced. Unfortunately, the width of the vacuum groove and the depth required for sufficient vacuum holding, border on the limits of current machine shop technology. Even with the thinnest vacuum groove producible, significant stresses are still developed in the wafer when subjected to a high pressure environment as well as the vacuum applied to the semiconductor wafer.
What is needed is a method and apparatus for minimizing leakage between the underside of the semiconductor wafer and the surface of the vacuum chuck, such that the wafer is sufficiently held onto the chuck surface. What is also needed is a method and apparatus for holding the semiconductor wafer during the supercritical processing which does not break the semiconductor wafer or otherwise damage the semiconductor wafer when placed on the vacuum chuck.